Strip transmission filter

ABSTRACT

A strip transmission filter having resonators with impedance skips in which the resonators are provided in a ceramic substrate. The strip transmission filter also includes capacitive couplings. The ceramic substrate has a metallic coating on all sides except for a face side. The coupling structures are at the face side of the substrate. The ceramic substrate is constructed in a stepped formation in a region of the coupling structures and at least one ground terminal. Thus, the adhesion of metallic coating is increased which, in turn, facilitates soldering.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a strip transmission filter with resonatorsthat are provided in a ceramic substrate. In particular, the inventionrelates to a strip transmission filter having capacitive couplingstructures for coupling an HF signal in and out, and in which theceramic substrate has a metallic coating on all sides with the exceptionof the face side, which has the coupling structures.

DESCRIPTION OF RELATED ART

Known strip transmission filters typically have a capacitive coupling ofthe resonators. On the base surface of a ceramic substrate, one surfaceis separated from the surrounding mass galvanically, so that thissurface forms a capacitance with repsect to a track that is arrangedabove on the ceramic substrate and is separated by the dielectric.

The capacitance is dependent on the dielectric constant ε of thedielectric, the thickness of the substrate, and the size of the surface.

European Document No. 718 906 A1 teaches a ceramic substrate having twostrip transmission line resonators arranged on its top surface. By agalvanic separating of the all over ground metallization, metal surfacesfor coupling are created on the bottom side of the substrate. On the topside of the ceramic substrate with the stripline resonators, metallicsurfaces are arranged, which are contacted to the metallic surfaces forcoupling with the aid of through holes which are metallized on theinside.

The coupling of the strip transmission filter is accomplished bycontacting the metallic coupling surfaces, which are galvanicallyseparated from the surrounding ground metallization by the separatingsurfaces, through to the other side of the ceramic substrate. Thus, thecapacitive coupling with the strip transmission resonators on theopposite side is achieved.

The coupling is determined mainly by the spacing of the couplingstructures, and not by the thickness of the substrate, which preferablyconsists of a highly dielectric microwave ceramic. Thus, the couplingstructure can be produced mechanically or by etching with significantlyhigher tolerances. Furthermore, the coupling can also be created byetching, whereby the position of the mask of photosensitive resist isnot critical to the coupling capacity.

A disadvantage of the above strip transmission filter is that, it can beproduced only at a relatively great expense. Another disadvantage isthat the dimensions of the component are also increased due to themeasure of through-contacting that is taken in order to connect themetallic surfaces that are provided on both surfaces of the striptransmission filter to each other.

Furthermore, the pass characteristic of microwave ceramic filters shouldhave an optimally high edge steepness in order to suppress LO oscillatorfrequencies, image frequencies, etc. At the same time, however, the polenumber—that is, the number of inner conductors—should be optimally smallin order to obtain an optimally low insertion loss (which will bedetailed later), since this rises with the number of poles.

Microwave ceramic filters were used for this, in which the couplingstructures are arranged in the region of the outer edges, and aone-sided steepening of the filter curve edges was achieved by impedanceskips of λ/4 resonators that are to be coupled to one another.

SUMMARY OF THE INVENTION

It is the an object of the present invention to produce a striptransmission filter which comprises a better adhesion of the metalliccoating on the ceramic substrate. A further object is to be able toadapt the bandwidth of the filter to the respective applications withoutnotable outlay.

In an embodiment, the present invention provides a strip transmissionfilter having a ceramic substrate having a face side. A metallic coatingis on the ceramic substrate, but not on the face side of the substrate.Resonators are provided in the ceramic substrate, the resonators eachhave an impedance skip. Capacitive couplings are on the face side andprovide a HF signal in and out of the strip transmission filter. Atleast one ground terminal is on the face side. The ceramic structure isstepped in a region of the capacitive couplings and the at least oneground terminal.

In an embodiment, the strip transmission filter includes a step depth ofthe at least one ground terminal that can be adjusted for the purpose ofchanging a bandwidth of the filter.

In an embodiment, the strip transmission filter has an edge steepnessthat provides a filter curve, F. The filter curve, F, is adjustable bychanging a spacing, A, between the capacitive couplings and a size ofeach of the capacitive couplings.

According to a preferred embodiment of the invention, a gradation isrealized in the ceramic substrate in which the strip transmissionresonators of the strip transmission filter are arranged, said stepbeing realized in the region of the capacitively acting couplingstructures and in the region of at least one ground terminal.

Such an inventively provided step in the region of the couplingstructures increases the adhesion of the metallic coating, and allowsfor more effective soldering of microwave lines that are to be connectedthrough this.

According to an advantageous development of the invention, the depth ofthe step—that is, the dimension of the step in the direction of thelongitudinal extent of the striplines—of a ground terminal that isprovided in the region of the coupling structures can be adjusted. Thismakes it possible to correct the bandwidth of the pass characteristic ofthe strip transmission filter in a simple but effective manner.

Furthermore, according to another advantageous development of theinvention, the edge steepness of the filter curve can be correspondinglyadjusted in view of the respective application by varying the distancebetween the coupling structures and/or varying the size of the couplingstructures.

In practice, this means that when the spacing between the couplingstructures is small and the coupling itself is selected to be weak (forinstance, by constructing the coupling structures very narrow), the edgesteepness of a strip transmission filter of such a construction issignificantly increased due to an overcoupling between the couplingstructures.

The invention is detailed below with reference to the enclosed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged schematic perspective view of a preferredembodiment of an inventive strip transmission filter.

FIG. 2 is a sectional view along a line II—II in FIG. 1.

FIG. 3 is a graph representing the inventively achievable steepening ofa filter curve's front edge according to the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

In FIG. 1, a strip transmission filter, referenced SF as a unit, has aceramic substrate 1, in which two strip transmission resonators 2 and 3are provided, whose course inside the substrate 1 in FIG. 1 is merelyindicated by broken lines. The course of the strip transmission linescan be derived more exactly from the sectional view in FIG. 2, whichwill be discussed later.

On a top side 10 of the ceramic substrate 1 and on the side of the frontface 12 are coupling structures 7 and 8. Between and outside thecoupling structures 7 and 8 are ground terminals 4 to 6 which areproduced by galvanic separation of the metallized surface 10, forinstance, mechanically, by etching, or with photosensitive resist.

Apart from the coupling surfaces 7 and 8 and the front face 12, theceramic body 10 is metallized on all sides. That is, even the surfaceopposite the front face 12, in which the two strip transmission lines 2and 3 end, is metallized.

As shown in FIG. 1, the two coupling structures 7 and 8 and the groundterminal surfaces 4 to 6 comprise a gradation 11. Furthermore, a smallerstep 13, which recedes even further back, is constructed along the lowerlongitudinal edge 12′ of the front face 12.

As already mentioned, the two steps 11 and 13 improve the adhesion ofthe metallic coating, particularly in the region of the face side 12 andin the region of the transitions from the top side 10 to the face side12, or respectively, from the face side 12 to the bottom side of thesubstrate 1. The construction of steps also facilitates the soldering oflines quite appreciably.

FIG. 2 shows a sectional view of the embodiment in FIG. 1. In FIG. 2,the two strip transmission lines 2, 3 include line discontinuities 2′,or respectively, 3′, which create the inventively provided impedanceskips on the basis of surge impedance skips.

The line discontinuities 2′ and 3′ in FIG. 2 are such that the spacingbetween the inner longitudinal edges 20 and 30 of the strip transmissionlines 2, or 3, respectively, in the region of the coupling structures 7and 8 is smaller than the spacing between the corresponding longitudinaledges 21 and 31 in the rear part of the substrate 1, or respectively, inthe upper part of the sectional illustration of FIG. 2.

In general, it is possible to influence the edge steepness of a striptransmission filter by means of the line discontinuities 2′ and 3′represented in FIG. 2, by displacing the sections of the striptransmission lines 2 and 3 situated inside the substrate 1 relative tothe left and right side surfaces 14 or 14′. In the inventive striptransmission filter SF, the front edge of the pass characteristic can beadditionally steepened by reducing the spacing A between the twocoupling structures 7 and 8 (see FIG. 1) and/or additionally selecting aweak type of coupling; that is, by realizing the two coupling structures7 and 8 so as to be very narrow, for example.

On the basis of the overcoupling between the coupling structures 7 and 8which this creates, the steepness can be increased appreciably, as isillustrated by the graphs in FIG. 3. In FIG. 3, the frequency is plottedon the abscissa, and the amplitude is plotted on the vertical axis.

By reducing the spacing A between the coupling structures 7 and 8 and/orby designing very narrow coupling structures, in a strip transmissionfilter the front edge VF2 of its pass curve FK2 can be inventivelysteepened appreciably compared to the front edge VF1 of the filter curveFK1 of a strip transmission filter in which the above describedinventive measures have not been taken.

In the diagram of FIG. 3, the signal curve is plotted in the form ofamplitude as a function of frequency. Relative to a damping 0, which isindicated in FIG. 3 by a horizontal reference line, the payload signalthrough the strip transmission filter SF experiences a definiteunavoidable damping, which is a matter of the above mentioned insertionloss. This insertion loss, referenced ED in FIG. 3, is not impaired,particularly not worsened, by the steepening of the front edge VF2 ofthe pass curve FK2.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present inventionwithout diminishing its attended advantages. It is, therefore, intendedthat such changes and modifications be covered by the appended claims.

I claim:
 1. A strip transmission filter, comprising: a ceramic substratehaving a face side; a metallic coating on the ceramic substrate, but noton the face side; at least two resonators being provided in the ceramicsubstrate, the at least two resonators each having an impedance skip; atleast two capacitive couplings on the face side, the at least twocapacitive couplings provides a HF signal in and out of the striptransmission filter; at least one ground terminal on the face side; andthe ceramic structure being stepped in a region of the at least twocouplings and the at least one ground; said at least one ground terminalhaving a step with a step depth to set a bandwidth of said filter.
 2. Astrip transmission filter according to claim 1, wherein said capacitivecouplings each has a size and wherein said capacitive couplings having aspacing therebetween, and wherein said filter has a filter curve that isset by a selection of said sizes of said capacitive couplings and saidspacing between said capacitive couplings.